As you clean out the tank every weekend, I cant understand the smell. Im assuming you clean the filter and pads every weekend too? Its possible the filter you have isnt enough for the size tank. Its also possible that the motor part on the filter has become clogged and thats why the water is becoming discoloured again so quick. If you have a look, you may be able to see where you can take the filter apart a little to clean it inside too....be careful though, dont force anything apart or it may break.
Another thing to check is the stones at the bottom.......do they need changing/cleaning? If old food and bits have become trapped in the stones and rotted, this may be why it is smelling?
Make sure the stones/bottom of the tank is clean, make sure the whole filter is clear, not just the filter pad, try replacing 2/3 of the water, each time you clean it out, with treated water.
What type of fish do you have?